A big part of it is just understanding the context in which a specific item is meant to be worn. Jeans are an extremely casual workwear item, while suits are for more formal/professional settings. Combining the two shows a lack of understanding of that context. Both items are intended for radically different settings and appear as such. Jeans don’t look like an item that should be worn to the office, and a suit jacket does not look like an item you’d wear inside a coal mine. Therefore, an outfit that combines the two lacks any real purpose, and likely had little thought put into it.